As well known, in a semiconductor manufacturing process, the concentration of a chemical used in each processing step affects the quality of semiconductor devices, and therefore the state of a change in the concentration is constantly monitored using an absorption spectrometer to manage so as to keep the concentration of the chemical constant.
Meanwhile, as a conventional absorption spectrometer, for example, as an absorption spectrometer disclosed in Patent Literature 1, there is one including: a light source; a light detector adapted to detect light emitted from the light source; and a sample cell and reference cell that can be switched to each other and alternately arranged either a measurement position positioned in a light path of the light passing between the light source and the light detector or a retracted position retracted from the measurement position. In addition, the absorption spectrometer is adapted to detect, by the light detector, light passing through a dimming element contained in the reference cell in a state of arranging the reference cell in the measurement position (reference measurement), also detect, by the light detector, light passing through a sample (a chemical) contained in the sample cell in a state of arranging the sample cell in the measurement position (sample measurement), and on the basis of pieces of light amount data on the lights detected in the both types of measurement, measure the concentration of each component contained in the sample.
On the other hand, improvement in the performance of semiconductor devices is directly linked with miniaturization. For this reason, in recent years, in order to achieve further miniaturization, various types of chemicals used in a semiconductor manufacturing process have tended to be diluted. However, as the concentration of a chemical is decreased, a variation of the concentration more significantly affects the quality of semiconductor devices as compared with the case where the chemical concentration is high. Accordingly, in order to make it possible to accurately measure even the concentration of a diluted chemical, further improvement in the accuracy of an absorption spectrometer has been demanded.
Therefore, the present inventor has examined a conventional absorption spectrometer in detail in order to respond to the demand. As a result, the present inventor has found the possibility that a variation in reference light amount, which has been thought to be caused by a variation in the light amount of light emitted from a light source, is significantly contributed to by another cause.
A description will be given in detail.
The conventional absorption spectrometer employs a moving mechanism adapted to, when moving a reference cell between a measurement position and a retracted position, slide the reference cell along a guide bridging between the both positions.
Using the conventional absorption spectrometer, an experiment that every 10 minutes, reciprocated the reference cell between the measurement position and the retracted position, and rearranged the reference cell in the measurement position to measure the absorbance of the reference cell was performed. As a result, as illustrated in an experimental result in FIG. 3, it was confirmed that in every reference measurement cycle, the absorbance largely varied. The present inventor has first known that the variation is mainly caused by a large variation in the measurement intensity of reference light. Note that in this experiment, light emitted from a light source was converted into parallel light through a collimator optical system and then passed through the reference cell. Also, the normalized absorbance in FIG. 3 was obtained by performing normalization, i.e., making the maximum deviation from the average value of absorbances in the conventional absorption spectrometer equal to 1. Further, the absorbance in the present embodiment was obtained by after the same normalization as above, adding 2 for ease of viewing a graph.
For now, the specific mechanism why the measurement intensity of the reference light fluctuates every time the reference cell is moved is not clarified.
However, the present inventor is considering that when a reflection type dimming element having a small time-dependent change was used in place of an absorption type dimming element conventionally used in a reference cell, a variation in the light amount of light detected in every reference measurement cycle more remarkably appeared, and therefore every time the reference cell is moved and rearranged, the attitude of the dimming element may slightly shift to cause the light amount variation of the reference light. Specifically, the present inventor is considering that part of the cause for the light amount variation is that the effect of multireflection caused by reflection lights generated at the incident and emitting surfaces of the dimming element may change, and as a result, the light amount of light detected in every reference measurement cycle, which is supposed to be constant, may vary every time the reference cell is positioned in the measurement position.